This invention relates to pressure transducers, in particular to miniature pressure transducers provided on a silicon wafer able to operate at high temperatures.
Pressure transducers etched or “micromachined” onto a silicon wafer are significantly smaller than conventional pressure transducers. Because of their small size, such micromachined pressure transducers are able to be used in situations that would be impractical for a conventional sensor such as, for example, in particular applications in the aereospace, medical or automotive industries.
U.S. Pat. No. 6,629,465 discloses the preparation of pressure transducers from a silicon wafer. Such sensors involve etching a cavity into a silicon wafer to expose a thin silicon diaphragm supported over a silicon frame. The silicon diaphragm may be exposed to a pressure to be measured and will flex in a predetermined manner depending upon the pressure to which it is exposed. The flexing of the diaphragm can be measured using piezoresistors formed on the diaphragm by processes such as implantation and/or diffusion. The piezoresistors will be electrically connected to suitable control circuitry, possibly via contacts typically arranged around the diaphragm.
It is desirable to further reduce the size of a pressure transducer, without reducing the size of the diaphragm which would reduce its sensitivity.
Furthermore, pressure sensors etched onto silicon wafers are generally limited by the temperature range at which they can operate. One reason for this is that semiconductor junctions of tracks and piezoresistors doped into the silicon produce significant leakage currents at increasing temperatures degrading the signals received from the piezoresistors.
In order to prevent fluid being tested from coming into contact with electrical leads and contacts of the transducer, the transducer is often provided inside a metal diaphragm with non-compressible packing material such as oil provided between the transducer and the metal diaphragm. However, such an arrangement is also limited by the temperature at which it can operate because at increasing temperatures the oil expands interfering with flexing of the diaphragm and reducing the precision of measurements produced. Furthermore, the silicon sensor is generally attached using an adhesive which cannot withstand the high temperatures, either failing in strength or outgassing reducing the precision in the oil filled package.